Ion accelerators with closed electron drift, also known as “Hall effect thrusters” (HETs), have been used as a source of directed ions for plasma assisted manufacturing and for spacecraft propulsion. Representative space applications are: (1) orbit changes of spacecraft from one altitude or inclination to another; (2) atmospheric drag compensation; and (3) “stationkeeping” where propulsion is used to counteract the natural drift of orbital position due to effects such as solar wind and the passage of the moon. HETs generate thrust by supplying a propellant gas to an annular gas discharge channel. Such channel has a closed end which includes an anode and an open end through which the gas is discharged. Free electrons are introduced into the area of the exit end from a cathode. The electrons are induced to drift circumferentially in the annular exit area by a generally radially extending magnetic field in combination with a longitudinal electric field, but electrons eventually migrate toward the anode. The electrons collide with the propellant gas atoms, creating ions which are accelerated outward due to the longitudinal electric field. Reaction force is thereby generated to propel the spacecraft.
It has long been known that the longitudinal gradient of magnetic flux strength has an important influence on operational parameters of HETs, such as the presence or absence of turbulent oscillations, interactions between the ion stream and walls of the thruster, beam focusing and/or divergence, and so on. Such effects have been studied for a long time. See, for example, Morozov et al., “Plasma Accelerator With Closed Electron Drift and Extended Acceleration Zone,” Soviet Physics-Technical Physics, Vol. 17, No. 1, pages 38–45 (July 1972); and Morozov et al., “Effect of the Magnetic Field on a Closed-Electron-Drift Accelerator,” Soviet Physics-Technical Physics, Vol. 17, No. 3, pages 482–487 (September 1972). The work of Professor Morozov and his colleagues has been generally accepted as establishing the benefits of providing a radial magnetic field with increasing strength from the anode toward the exit end of the accelerator. For example, H. R. Kaufman in his article “Technology of Closed-Drift Thrusters,” AIAA Journal, Vol. 23, No. 1, pages 78–87 (July 1983), characterizes the work of Morozov et al. as follows:                The efficiency of a long acceleration channel thus is improved by concentrating more of the total magnetic field near the exhaust plane, in effect making the channel shorter. Another interpretation, perhaps equivalent, is that ions produced in the upstream portion of a long channel have little chance of escape without striking the channel walls. Concentration of the magnetic field at the upstream end of the channel therefore should be expected to concentrate ion production further upstream, thereby decreasing the electrical efficiency.Id. at 82–83. For experimental purposes, Morozov et al. achieved different profiles for the radial magnetic field at the exit end by controlling the current to coils of separate electromagnets. For a given magnetic source (electromagnet or permanent magnets), other ways to affect the profile of the magnetic field are configuring the physical parameters of magnetic-permeable elements in the magnetic path (such as positioning and concentrating magnetic-permeable elements at the exit end of the accelerator), and by magnetic “screening” or shunts which can be interposed between the source(s) of the magnetic field and areas where less field strength was considered desirable, such as near the anode. For example, in their paper titled “Effect of the Characteristics of a Magnetic Field on the Parameters of an Ion Current at the Output of an Accelerator with Closed Electron Drift,” Sov. Phys. Tech. Phys., Vol. 26, No. 4 (April 1981), Gavryushin and Kim describe altering the longitudinal gradient of the magnetic field intensity by varying the degree of screening of the accelerator channel. Their conclusion was that magnetic field characteristics in the accelerator channel have a significant impact on the divergence of the ion plasma stream.        
In addition to the traditional structure of a Hall effect thruster disclosed in the publications referred to above, there have been more recent attempts to increase thruster efficiency and life by providing systems with modified magnetic fields at the exit end of the thruster. One example is the device shown in Arkhipov et al. U.S. Pat. No. 5,359,258, which provides radially inner and outer sources of magnetic fields to produce the substantially radial field lines at the exit end, and an “internal magnetic screen” and “external magnetic screen” in combination with an anode retracted inward from the exit plane of the thruster to concentrate the magnetic field at the exit end and lessen the magnetic field adjacent to the anode. King et al. U.S. Pat. No. 6,208,080, discloses another magnetic field distribution at the exit end of an HET and a magnetic shunt system for achieving that distribution. In the King et al. design, the walls of the gas discharge-acceleration chamber can be electrically conductive and maintained at anode potential. Conductive anodes located close to the exit plane also have been proposed, such as in Gopanchuk et al. U.S. Pat. No. 5,798,602, and Semenkin et al. U.S. Pat. No. 5,838,120. Particularly where anodes have been located close to the exit plane, the adjacent part of the anode may be more or less concave, such as in the devices disclosed in the patents issued to Gopanchuk et al. and Semenkin et al., referred to above, and Arkhipov et al. U.S. Pat. No. 5,892,329.